1. Field of the Invention
The present invention generally relates to the art of electronic transistors, and more specifically to a high electron mobility field-effect transistor structure having a passivated donor layer.
2. Description of the Related Art
A modulation-doped field-effect transistor (MODFET), otherwise known as a high electron mobility field-effect transistor (HEMT), is an extremely fast three-terminal electronic transistor device which has broad applications in millimeter wave systems, satellite receivers and transmitters, as well as advanced radar and fiber-optic systems. However, problems including device passivation have affected the reliability of HEMTs to such an adverse extent that widespread practical implementation has not been heretofore possible.
A conventional HEMT structure 10 illustrated in FIG. 1 includes a substrate 12, channel layer 14, spacer layer 16, donor layer 18, Schottky layer 20, and cap layer 22 formed on the substrate 12. A source 24 and drain 26 are formed on the cap layer 22. A gate 28 is formed on the bottom of a recess 30 which extends through the cap layer 22 and at least partially through the Schottky layer 20. The spacer, donor and Schottky layers 16, 18, 20 respectively are typically formed of a "wide" bandgap semiconductive material such as aluminum indium arsenide (AlInAs) or aluminum gallium arsenide (AlGaAs). The channel and cap layers 14 and 22 respectively are formed of a "narrow" bandgap material which is lattice-matched to the wide bandgap material, such as gallium arsenide (GaAs) or gallium indium arsenide (GaInAs). A major problem with the conventional structure is that the wide bandgap material which forms the donor and Schottky layers includes a component (aluminum in the above exemplary materials) which is prone to oxidation at low temperatures. The reference numeral 32 designates areas in which the Schottky layer 20 is exposed to the atmosphere and oxidized thereby. The oxidation spreads through the Schottky layer 20 into the donor layer 18, causing substantial reduction of the channel current.
Oxidation can be prevented by forming a passivation layer of dielectric silicon nitride or silicon oxide over the entire surface of the device, as described in an article entitled "A New GaAs Technology for Stable FET's at 300.degree. C., by K. Fricke et al, in IEEE Electron Device Letters, vol. 10, no. 12, pp. 577-579 (Dec. 1989). The passivation layer hermetically seals the structure, preventing oxidation of the Schottky and donor layers. However, this compromises the performance of the device because of the extra capacitance created by the dielectric passivation layer. In addition, such passivation layers must be deposited at relatively high temperatures, which can degrade the performance of the device.